For starters, did you empty the cache? This tutorial will show you how.

Have you cleared the cookies? This article will show you how.

There is an context menu item of a tab to "Close Tabs to the Right and to "Close Other Tabs",

If you would like to recover the unrestored tab data, you could try this approach, assuming it hasn't been too long already:

If Firefox is still running:

You can open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Open Folder" button.

If Firefox is closed:

Windows: Type or paste the following into the Windows Run dialog or the system search box and press Enter to launch Windows Explorer:

%APPDATA%\Mozilla\Firefox\Profiles

In that folder, do you see a semi-randomly-named folder? If so, click into it. If you find multiple such folders, find the one that was most recently updated.

Mac: See this article: Profiles - Where Firefox stores your bookmarks, passwords and other user data.

Back up session history files

In your profile folder, scroll down and double-click into the sessionstore-backups folder. The kinds of files you may find among your sessionstore files are:

• recovery.js: the windows and tabs in your currently live Firefox session (or, if Firefox crashed at the last shutdown and is still closed, your last session)
• recovery.bak: a backup copy of recovery.js
• previous.js: the windows and tabs in your last Firefox session
• upgrade.js-build_id: the windows and tabs in the Firefox session that was live at the time of your last update

Note: By default, Windows hides the .js extension (I don't know if this is an issue on Mac). To ensure that you are looking at the files I mentioned, you may want to turn off that feature. This article has the steps: https://www.bleepingcomputer.com/tutorials/how-to-show-file-extensions-in-windows/

I suggest making a copy of the contents of this folder in a safe place, such as your Documents folder.

Inspect session history files

To see a file's contents, drag and drop it from Windows Explorer (or on Mac, Finder) onto the textbox on the following page and click Scrounge URLs:

https://www.jeffersonscher.com/res/scrounger.html

The Save List button will create an HTML page you can use to launch links to your session history in whatever browser you prefer. Does it show the true URLs?

I didn't try LeeU's suggestions because John scher's suggestions always work for me, and this did work again.

Cor-el, I would have tried your suggestion to just close all those tabs, but I have tabs down the side, and I don't know which direction is "right." :(

John scher: thanks a big bunch! I got most of my lost tabs back.